1. Field of the Invention
The present invention relates to a piezoresonator, and more particularly to a piezoresonator with a built-in capacitor to be employed in an oscillation circuit, a filter circuit or the like.
2. Description of Related Art
A piezoresonator which has, on a piezoelectric substrate, a plurality of oscillation sections and a coupling capacitor section for electrically connecting the oscillation sections has been conventionally known. In the piezoresonator, if the portion of the piezoelectric substrate which holds the coupling capacitor section is polarized, when the coupling capacitor section is impressed with a voltage, the coupling capacitor section vibrates because of electrostrictive effect. Thus, the coupling capacitor section acts as a vibrating section, which lowers the electrical characteristics of the piezoresonator.
The following two measures to avoid the problem have been conventionally taken:
(1) the portion of the piezoelectric substrate which holds the coupling capacitor section is entirely left unpolarized; and PA1 (2) the piezoelectric substrate is entirely polarized, while the coupling capacitor section is subjected to damping processing, such as, heaping solder on the capacitor electrodes of the coupling capacitor section and coating the capacitor electrodes with resin.
However, the measures (1) and (2) have disadvantages. A piezoresonator processed in the way of (1) has a small insertion loss but is poor in the flatness of group delay characteristic. A piezoresonator processed in the way of (2) is good in the flatness of group delay characteristic but has a large insertion loss.